In ensuring the uniformity of the physical properties of a given fiber reinforced thermoplastic composite, it is necessary to determine whether a given sample of composite material does indeed contain fibers of a specified length. In the past, this was accomplished by burning off the thermoplastic resin portion of the composite, leaving behind the reinforcing fibers. The reinforcing fibers would then be picked up using an electrostatically charged brush, typically fashioned of camel hair, and placed on a sampling surface such as a Petri dish. Those fibers deposited upon a predetermined portion of the Petri dish would then be counted and their sizes measured. This process would be repeated until a predetermined number of fibers had been counted and measured, thereby permitting the calculation of an average fiber length.
As can be appreciated, this statistical sampling method is time consuming and yields only approximate results at best. More often, the results yielded by this sampling method skew the test results such that the average fiber lengths are reported as being shorter than what actually exists in the sample of composite material. Typically, this is due to breakage of the reinforcing fibers that had been embrittled in the burn-off process in which the fibers were removed from the thermoplastic resin. In addition, because the electrostatic charges present within a camel hair brush commonly used in this process can vary from moment to moment and day to day depending on many environmental factors including humidity, the size and number of reinforcing fibers that may be picked up in a given sampling would not be uniform. As can be appreciated, when the electrostatic forces on the camel hair brush are relatively strong, longer fibers would be picked up by the brush. Conversely, where the electrostatic charge on the brush was relatively low, only shorter fibers would be picked up. Therefore, the results of this prior art type of statistical sampling are variable and tend to report smaller average fiber lengths than what exist in reality.
In order to overcome these problems, the present invention comprises a burn-off procedure in which the reinforcing fibers are annealed, thereby preventing fiber embrittlement that might otherwise increase breakage during the testing process. In addition, the present invention comprises a fractionation process and apparatus that accounts for all of the fibers in a given sample of composite material. In this manner, more accurate results may be obtained. In addition, the process and apparatus of the present invention is faster and easier to use.